The present invention relates to a process for preparing 4-pentenoic acid, at least comprising the oxidation of a mixture (G) comprising 4-pentenal, 3-methyl-2-butanone and cyclopentene oxide, and to the use of a mixture (G) comprising 4-pentenal, 3-methyl-2-butanone and cyclopentene oxide for preparing 4-pentenoic acid. In the context of the present invention, the mixture (G) is preferably obtained as a by-product of the oxidation of cyclopentene to cyclopentanone by means of dinitrogen monoxide.
4-Pentenoic acid and its esters find use as odorants and flavorings, especially in milk and cheese products. In addition, 4-pentenoic acid and 4-pentenoates are also known as pharmacologically active substances which are capable of inducing hypoglycemia (see, for example, H. Sherratt, H. Osmundsen, Biochemical Pharmacology (1976) 25(7), 743-750).
It is known from the prior art that 4-pentenoic acid can be prepared by reacting diethyl malonate with allyl chloride in the presence of a base and subsequent hydrolysis, decarboxylation and acidification (see J. Xie, B. Sun, S. Sha, F. Zheng, W. Dang, Beijing Gongshang Daxue Xuebao, Ziran Kexueban (2007), 25(1), 7-9). However, this synthesis has the disadvantage that the reaction proceeds from relatively expensive feedstocks and utilizes them only poorly (loss of CO2). At a described yield of approx. 70%, about 340 kg of reactants, namely diethyl malonate and allyl chloride, are required to prepare 100 kg of 4-pentenoic acid.
The prior art additionally discloses the principle of the oxidation of unsaturated aldehydes with oxygen to give the corresponding carboxylic acids. For example, the oxidation of alpha,beta-unsaturated aldehydes is described. For example, WO 2008/017342 describes the oxidation of citronellal to citronellic acid with oxygen in the presence of a supported gold catalyst. U.S. Pat. No. 4,097,523 describes the oxidation of alpha,beta-unsaturated aldehydes with oxygen in the presence of thallium as a catalyst. Other homogeneous catalysts, for example manganese, copper or cobalt, are also known in the oxidation of alpha,beta-unsaturated aldehydes with oxygen (“Crotonaldehyde and Crotonic acid”, R. P. Schulz, J. Blumenstein, C. Kohlpaintner in “Ullmann's Encyclopedia of Industrial Chemistry”, 7th edition, online Release 2008). However, none of these prior art documents provides any indication that these methods are applicable to the oxidation of alpha,beta-unsaturated aldehydes to other unsaturated aldehydes.
Proceeding from this prior art, it was an object of the present invention to provide a process of economic interest for preparing 4-pentenoic acid.
It has been found that, surprisingly, 4-pentenoic acid can be prepared in good yields and high purity by the oxidation of 4-pentenal.
A significant factor in the economic viability of the oxidation process is the availability of 4-pentenal. 4-Pentenal can be prepared, for example, by Claisen isomerization of allyl vinyl ether (see, for example, R. F. Webb, A. J. Duke, J. A. Parsons, J. Chem. Soc. (1961), 4092-4095). However, allyl vinyl ether is not a commercially available vinyl ether and is thus not readily available industrially either.
Alternatively, 4-pentenal can also be prepared by the thermolysis of acetaldehyde diallyl acetal, as described in DE 25 17 447. However, this method has the disadvantage that allyl alcohol has to be recycled, a significant proportion of which decomposes to propionaldehyde, which makes the process uneconomic in turn.
The prior art further discloses that 4-pentenal forms as a by-product in the preparation of cyclopentanone by oxidation of cyclopentene with dinitrogen monoxide (N2O) (see, for example, E. V. Starokon', K. S. Shubnikov, K. A. Dubkov, A. S. Kharitonov, G. I. Panov, Kinetics and Catalysis (2007), 48(3), 376-380). However, the 4-pentenal is not obtained as a pure substance but as a mixture with other by-products from the cyclopentanone synthesis.